Workaround:

We have a fix for this in the Visual F# nightly release, and the fix will also be available in a future update. Closing and re-opening an affected file will also fix it for that file, though this is just temporary.

There are multiple issues related to F# support for .NET Core and .NET Standard projects, which we do not consider fully supported. We're currently working on full support. We wish to call out those issues here, though, in case you do choose to load these types of projects.

This problem has been fixed, and will be in a forthcoming servicing release. In the meantime, it is also fixed in the F# Nightly releases, documented here: https://blogs.msdn.microsoft.com/dotnet/2017/03/14/announcing-nightly-releases-for-the-visual-f-tools/

You can use the nightly release if you wish to use VS 2017 15.3 with this fix applied before the servicing release.